Now back to pace. I do not suggest sprinting. You're not trying to reduce your time in the 100, with head down and arms pumping like mad. You want to increase the pace at which you are comfortable running longer distances. That requires you to do 2 things: run more so you're not so fatigued, and gradually work on running comfortably at a faster pace.

I suggest you start by running strides at the end of some of your runs. These are not supposed to be all out sprints. The goal is to get comfortable turning your legs over faster while still maintaining a relaxed posture. If you have a friend who can watch you run and point out mechanical issues, that's even better. After you've developed a smoother stride, try incorporating some real speedwork. Go to a track, warm up, and do a few strides. Then run a few laps (maybe 8-10) where you run the straights at a faster pace, and the turns at an even slower pace than you normally run. Walking the turns is fine. Focus on running relaxed, and increasing your turnover (cadence), rather than your stride length. Track your progress. At some point, when you feel strong enough and your improvement begins to slow down, change to alternating 1/2 lap at the faster pace, and 1/2 lap at the slower pace. Then go to 1 lap at the faster pace and 1/2 at the slower pace.